Abstract : We have measured the lifetime of the superconductive state of a current biased Josephson junction shunted by a microwave circuit which can be treated as an electromagnetic echo line. By varying the length of this line, we can adjust the total reflection time of an echo. At low temperature (T=20mK), the junction switches out of the superconductive state by macroscopic quantum tunneling. We show that this effect is greatly reduced if the total reflection time of the echos is smaller than a new caracteristic time of tunneling, which is different from the lifetime. We interpret this new caracteristic time as the mean time spent by the particle under the potentiel barrier during its escape from the well. At
higher temperature (T=1K), the junction switches out of the superconductive state by thermal activation. In this regime, the escape rate varies in an oscillatory fashion with the length of the line. We interpret this phenomenon as the manifestation of phase difference oscillations, sustained by thermal fluctuations, of the junction in the superconductive state before the transition toward the dissipative state.